1. Field of the Invention
The present invention is directed to an ultrasound phase difference method for measuring high flow rates, particularly of the intake air for motor vehicle drives, by a measuring arrangement which is formed by two ultrasound measuring paths intersecting in the flow axis of a measuring tube. The measuring paths each defined between a transmission ultrasound transducer and a receiving ultrasound transducer, the ultrasound transducers being arranged in the wall of the measuring tube so that one of the measuring paths has its transmission ultrasound transducer upstream from its receiving ultrasound transducer and the other measuring path has its transmission ultrasound transducer downstream of its receiving ultrasound transducer. The measuring arrangement also includes an oscillator, at least one receiving amplifier and an evaluation device.
2. Description of the Prior Art
As is known in the art, the relation, ##EQU1## is valid for ultrasound flow rate meters based on the phase difference method, whereby .DELTA..phi. is the measured phase difference, f is the operating frequency, l is the effective length of the measuring path, v is the effective flow rate and c is the speed of sound in the fluid.
It is desirable for achieving high measuring sensitivity to select the ratio .DELTA..phi./v as large as possible by a corresponding dimensioning of the measuring path. This, however, would cause the unambiguity range of the phase discriminator to be exceeded given high flow rates. The absolute phase difference is thus not measured; rather, an incremental .DELTA..phi.' is measured which is obtained from the absolute phase difference by subtracting 2n.pi. or n.pi. dependent on the type of phase discriminator employed), i.e. EQU .DELTA..phi.'=.DELTA..phi.-2n..pi. EQU or EQU .DELTA..phi.'=.DELTA..phi.-n..pi.
The whole-number factor n is generally unknown. Disturbing modes of the phase discriminator occur at the locations .DELTA..phi.=2n.pi. (or .DELTA..phi.=n.phi.).
Conventional measuring formats therefore had to be dimensioned such that the measured phase difference lies within the high precision range of the phase discriminator, even given maximum flow rate. This constitutes a restriction of the possibilities in the design of the measuring path and in the possible measuring sensitivity.
In particular, ultrasound flow rate measuring paths for the intake air of motor vehicle engines can only be dimensioned by selection of a low ultrasound frequency because of the high maximum flow-through rate (v.sub.max .apprxeq.400 m.sup.3 /h, .phi..apprxeq.60 mm) and of the low speed of sound in air. The utilization of low frequencies, however, causes problems due to, among other things, the high noise level at low frequencies.